<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <h1 id="msg">123</h1>
  <button id="btn">change</button>
</body>
<script>
  let massage = 'hello react'
  const msg = document.querySelector('#msg')
  msg.innerHTML = massage
  const btn = document.querySelector('#btn')
  btn.addEventListener('click',() => {
    msg.innerHTML = 'hello nnn'
  })
</script>
</html>